Method of plasticizing rubber



Patented Apr. 18, 1950 2,504,903 mz'rnon or rrns'rrcrzmc comma Bernard Miller Stnrgis, Pitman, N. 1., and John Joseph Verbanc, Wilmington, DeL, assignor's to j E. I. du Pont de Nemours & Company, Wib

mington, Deb, a corporation of Delaware No Drawing. Application October 1'1, 1947,

Serial No. 780,159

(or Eco-101 4 Claims.

, This inventionrelates to rubber and more particularly to treating rubber to produce a more plastic product.

It is well known that rubber when subjected to mechanicalv working in the presence of air or oxygen is rendered more plastic. The degree of plasticization is determined to a great extent by the time and temperature of milling. When milled for a suflicient length of time, rubber 1 becomes very soft and at the same time will lose i much of its ability to vulcanize. In order to reduce the time of milling and to prevent the deteriorating action of continued milling, it is common practice to add to the rubber, during the milling operation, certain materials vwhich assist in producing a more plastice and workable product. The materials which are commonly used are oils, esters, waxes, fats, alcohols, acids, resins and the like, which assist in the production of soft rubber either by a swelling action on the rubber or by acting as a lubricant. Some of the more common softeners are mineral oil, pine tar, palm oil, rosin oil, dibutyl phthalate, paraifin wax, glycerine and stearic acid. Relatively large amounts of these agents must be used to produce the desired plasticity. The presence of these so called physical softeners" leads to the impairment of a number of the physical properties of the resulting vulcanizates and for this reason their use is often undesirable.

It is the object of the present invention to to rubber during processing will materially decrease the time necessary for breakdown of the rubber and will decrease the overall time neces-e sary for addition of compounding agents. A further object is to provide organic compounds which when added to rubber will increase the plasticity of the raw product to produce better molding properties and. superior extrusion characteristics. Another object is to produce rubber which may be milled at relatively low temperatures, 50 to 175 0., making design of new and costly equipment unnecessary. Further, it is the object of this invention to provide organic compositions which when added ,'to cements will materially decrease the viscosity making possible the production of coating compositions of higher solids content. A still furtherobject is to provide a method of plasticizing rubber which will not lead to interior properties in the resulting vulcanizates. Further objects will appear hereinafter.

The above and other objects may be accomplished in accordance with our invention which comprises subjecting unvulcanized rubber to the action of 0.05% to 5% by weight based on the rubber of a-thienyl mercaptan (a-thiophene mercaptan), its corresponding disulfide which is a,-d ithienyl disulflde, its zinc salt, and its esters derived from carboxylic acids containing 1 to 8 carbon atoms. Among such carboxylic acids are formic, acetic, monochloroacetic, dichloroacetic,

provide organic compositions which when added trichloroacetic, butyric, caprylic, benzoic, p-nitrobenzoic, o-chloro bentoic, furoic, oxy'lic, suc'cinic, adipic, suberic, o-toluic, and phthalic acids.

These plasticizers may be produced by any methods known to those'skilled in the art. By way of illustration, a-thienyl mercaptan may be made by reacting c-bromo-thiophene with ma:- nesium in dry ether, treating the resulting Grignard reagent with free sulfur and decomposing the resulting magnesium salt of the mercaptan with acid. An analogous method for making thiophenols is described in Beilstein, vol. 6, page 294. The thienyl mercaptan derivatives used in the present invention may be made by methods analogous to those used, with thiophenols. Thus the disulflde may be made from the mercaptan by gentle oxidation, the zinc salt by heating the mercaptan with zinc oxide in an alcoholic solvent, and the ester, for example, I; treating the mercaptan with the appropriate ac chloride or anhydride.

The unvulca'nized'rubber is subjected to the action of the above compounds for a suflicient length of time for the said compounds to effect a marked decrease in the resistance of the rubber to flow over that which the rubber would have ifsubjected tothe same conditions in the absence of the said softeners.

By the term a-thienyl" we mean the heterocyclic radical represented by the formula:

Thus the formulas for the above plasticizers are:

c-thienyl mercaptan Zinc salt of a-thienyl mercaptan acetic ester of a-thienyl mera,a-dithienyl disulflde captan as an example of the carboxylic acid esters a Gordon plasticator, Banbury or Werner and In order to more clearly illustrate our inven- (3) Produces stocks which extrude and mo tion, preferred modes of carrying the same into better. j effect, and advantageous results to be obtained (4) Makes possible the production of superior thereby, the following examples are given: vulcanizates since the oils. tars, bitumins etc..

, I formerly employed as softening aids can be elim- EKAMPLE 1 inated.

' The tremendous softening effect produced by 75' grams of smoked sheet rubber was placed 43 1 mercaptan is completely destroyed on in a small Werner 81 Pflelderer mixer and the vulcanization so t t t vulcanizate produced is ch r e mix d f /2 minute. The a e to be 10 superior to that obtained by the addition 01 phytested Was then added in the quantity indicated sical softeners even though the degree of softenthe following table and mixed with the l'llbing is tremendously greater prior to curing her at the temperature and time indicated. The plasticized rubber sosobtained was removed from EXAMPLE 2 the mixer and sheeted out on a rubber mill. is Employing the same technique as described in Plasticity pellets were then cut from a sample of Example 1 the zinc salt, acetic ester and disulflde the plasticized polymer and the plast city and of o-thienyi mercaptan were evaluated as plastirecovery measured on a Williams parallel plate clzing agents. The data obtained is listed in Table plastometer (cf. Williams, Ind. Eng. Chem 16, II. Duplicate determinations are given under 862 (1924)). Data obtained using e-thienyl mer- 20 (1) and (2).

Team II Williams plasticity-recovery measurments T Plasticity Recovery Mixing Plasticizing Agent T211113" a a? ga None 100-100 None 103 140 01 00 Zinc a-thienyi mercaptide 100-100 10 0.20 04 0 -2 Do 100-100 10 0.00 10 70 1 -1 Do 100-100 10 1.00 00 00 -1 -a a,d-Dithlenyl 0101111100.. 100-100 .10 0.20 110 112 12 10 Do 100-100 10 0.00 100 100 4 0 Acetic ester of a-thienyl mercaptan 100-100 10 0.00 00 0s 0- -2 captan as the processing agent is recorded in EXAMPLE 3 Table I. Duplicate determinations are given under (1) and (2). 75 parts of unmilled smoked sheets was placed Tenn: I

Williams plasticity-recovery measurments Mm Plasticity Recovery 15 None 164 15 0. 80 -7 --6 16 l. 00 71 -7 -4 15 None 153 149 57 50 15 0. 25 5 5 15 0. 50 73 74 -8 8 l5 1. 00 67 70 -9 7 The information listed in Table I shows that in a small Werner 81 Pfleiderer mixer. 0.75 part a-thienyl mercaptan is a very strong softening of a-thienyl mercaptan was added and the charge agent for rubber. This tremendous softening mixed at 158-160" C. for 15 minutes. 20 parts of efiect is beneficial in that it 55 the resultant plasticized polymer was mixed with (l). Materlally decreases the power consump- 180 parts of xylene until a smooth cement was tion of the mill or mixer employed to masticato formed- The St rm r vis ty of th sm t and compound the polymer. cement was measured at 25 C. and found to be (2) Decreases the time required for the in- 225 seconds. Data obtained using other comoorporation of compounding agents. 60 pounds of this invention are recorded in Table III.

Tenn: III Stormer viscosity in seconds at 25 C.

Stormer Viscosity Mums Tum Pet I 10 Per Cent Plasticizing Agent Temp., in Cent 3 m sec.

'0. Min. Used Q None -100 10 None 4,000

Zinc a-thienyl mercaptide 158-160 15 0.25 980 Acetic esterofo-thienyl momaptam 158-160 15 0.5 110 .a-Thienyl msreaptan 158-160 15 0.5 478 Do 100-100 10 1.0 m0

EXAMPLE4- 30 grams of unmilled smoked sheet rubber was placed on a 2 x 6" rubber mill, through which wate at the indicated temperature was continually passed, and milled for one minute. 1% a.- thienyl mercaptan wasadded and the milling continued for an additional minutes. The placticized polymer was removed from the mill and tested for softness. Data obtained in this and garner similar experiments are recorded in Table TABLE IV Mill plasticization of natural rubber Williams Parallel Plate Mill Plasticizing Agent 1 Texan, Plasticity Recovery None (Control) 75 130 25 Do 100 179 65 125 191 74 7 5 94 91 1 3 100 90 93 4 3 125 84 88 5 3 75 107 110 6 3 100 112 1-20 7 9 D0 125 125 125 9 S Acetic ester of hienyl mercagtan i. 75 92 92 4 4 0 100 102 103 7 8 D0 125 82 95 4 4 1% 0! each agent was used except in the control.

Rubber cements of increased rubber content and decreased viscosity may be prepared by dis-.

solving the rubber in any suitable solvent-such as benzene, gasoline, carbontetrachloride, ethylene, dichloride and the like and treating the solution with a-thienyl mercaptan, its corresponding disulfide, its zinc salt, or its above named esters. The additive will act thereupon to reduce greatly the viscosity of the cement as shown in Example 3. It will generally be preferable either to treat the rubber with the agent to develop a soft product before dissolving the rubber in the solvent or to add the agent to the solvent before dissolving the rubber therein. By these last two methods it will be found that the solution of the rubber in the solvent will be greatly expedited and large amounts of rubber may be incorporated in the solvent to produce a cement of specified viscosity.

By the phrase decreasing the resistance of rubber toflow we mean to include decrease in the viscosity of the rubber solutions as well as increase in plasticity of ordinary unvulcanized rubber in a more or less solid state.

The plasticizing agents of this invention make possible the breakdown of rubber more completely and in a shorter time with an appreciable saving in power consumption and consequently a much greater output from each piece of equipment. The agents disperse well in rubber and not only soften rubber, but also improve the processing characteristics. In addition rubber plasticized with the materials of this invention dissolve more readily in organic solvents forming cements having a lower viscosity.

We claim as our invention:

1. The method of decreasing the resistance to flow of unvulcanized rubber comprising incorporating 0.05% to 5% by weight based on the unvulcanized rubber of a compound selected from the group consisting of unsubstituted a-thienyl mercaptan, its corresponding disulfide, its zinc salt, and its esters derived from carboxylic acids containing 1 to 8 carbon atoms.

2. The method of plasticizing unvulcanized rubber comprising incorporating 0.45% to 5% by weight based on the rubber of a compound selected from the group consisting of unsubstituted athienyl mercaptan, its corresponding disulfide, its zinc salt, and its esters derived from carboxylic acids containing 1 to 8 carbon atoms.

3. The method of decreasing the viscosity of I rubber cement comprising incorporating 0.05% to 5% by weight based on the rubber oi a compound selected from the group consisting of unsubstituted a-thienyl mercaptan, its corresponding disulfide, its zinc salt, and its esters derived from carboxylic acids containing" 1 to 8 carbon atoms.

4. Unvulcanized rubber containing as a plasticizing agent 0.05% to 5% by weight based on the rubber of a compound selected from the group consisting of unsubstituted a-thienyl mercaptan,

its corresponding disulflde, its zinc salt, and its esters derived from carboxylic acids containing 1 to 8 carbon atoms.

BERNARD MILLER STURGIS. JOHN JOSEPH VERBANC.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Certificate of Correction Patent No. 2,504,003 April 18, 1950 BERNARD MILLER STURGIS ET AL. It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

7 Column 1, line 16, for plastice read plastic; line 58, for a,a-dithienyl read a,a'-dithienyl column 6, line 22, for 0.45% read 0.05%; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 11th day of July, A. D. 1950.

THOMAS F. MURPHY,

Assistant aommiuioner of Patea'ta. 

1. THE METHOD OF DECREASING THE RESISTANCE TO FLOW OF UNVULCANIZED RUBBER COMPRISING INCORPORATING 0.05% TO 5% BY WEIGHT BASES ON THE UNVULCANIZED RUBBER OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF UNSUBSTITUTED A-THIENYL MERCAPTAN, ITS CORRESPONDING DISULFIDE, ITS ZINC SALT, AND ITS ESTERS DERIVED FROM CARBOXYLIC ACIDS CONTAINING 1 TO 8 CARBON ATOMS. 